This invention relates to a closed-loop or ring type communication system, and more particularly to an in-line coupling circuit incorporated in the closed-loop type communication terminal.
In department stores or supermarkets, Point of Sale (POS) system is used to efficiently process the clerical work such as sales price administration, stock administration, sales totalization. The POS system generally includes POS control or administration apparatus arranged in the central office of the store and a plurality of POS terminals arranged as electronic cash registers at the counters. The POS control apparatus and POS terminals are serially connected via communication line to form a closed loop. The POS terminal at each counter makes contact with the POS control apparatus in order to inquire the name and unit price of articles to be sold of the POS control apparatus and inform the POS control apparatus of sales information such as the number and sales amount of the sold articles. Further, when the POS control apparatus receives the sales information, it writes data into the file in order to perform the stock administration and sales totalization according to the received sales information.
Now, data communication between the POS control apparatus and POS terminals is explained in more detail. The POS control apparatus periodically generates a polling signal and gives opportunities to the POS terminals in the closed loop to transmit data. When a polling signal is transmitted from the POS control apparatus to one end of the communication line, it is fed through the closed loop, and transmitted from the other end of the communication line to the POS control apparatus. In this case, the POS terminal located in the most upstream position receives a polling signal generated from the POS control apparatus and transmits the polling signal to the POS terminal located in the downstream position. Thus, the polling signal is sequentially supplied to and transmitted from the POS terminals from the upstream to the downstream. Then, the polling signal supplied to the POS terminal located in the most downstream position is transmitted from the POS terminal to the POS control apparatus. A time slot between the two successive polling signals is divided into subslots which are respectively assigned to the POS control apparatus and POS terminals so that a frame of transmission data can be transmitted immediately after the preceding polling signal. When the POS control apparatus is required to make contact with any one of the POS terminals, it transmits transmission data at a time corresponding to the assigned subslot. Likewise, if the POS terminal is required to make contact with the POS control apparatus, it transmits transmission data at a time corresponding to the subslot assigned to the POS terminal. Further, each of the POS control apparatus and the POS terminals adds a GA character indicating the end of data frame to the end of the transmission data. In the prior art POS system, each of the POS terminals has an in-line display lamp which is lit when it receives the polling signal. When the power source of the POS terminal to be used is turned on, the in-line display lamp is lit to indicate that the POS terminal is set in the in-line coupling state in which it can be communicated with the POS control apparatus. The period in which the in-line lamp is kept lit is set slightly longer than the interval at which the polling signal is generated. Therefore, the in-line display lamp is kept lit in the case where the polling signal is repeatedly received. However, if the polling signal is interrupted, the in-line lamp is turned off. Therefore, the operator of the POS terminal can infer from the turn-off of the in-line display lamp that something abnormal occurs in the POS system.
In the POS system, if one of the POS terminals becomes defective, none of the other POS terminals can be operated at the same time, making the sales service confused. In this case, the in-line display lamps of the POS terminals locating in the downstream of the defective POS terminal are all turned off. Therefore, the operator of the POS control apparatus had to sequentially make contact with the POS terminals by telephone, for example, and check whether or not the in-line display lamp of each POS terminal is lit until the defective POS terminal is detected. Thus, it generally takes a long time to start the substantial restoring operation for the POS system.